The technology for wet scrubbing combustion effluents to remove SO.sub.x provides gas-liquid contact in a number of different configurations. Among the most prominent are the open countercurrent spray towers and towers which employ packings or trays. In the vast majority of these designs, gas flows vertically upward and the liquid flows downward, under the influence of gravity. The use of a variety of reagents has been suggested, but the most preferred are those which can be purchased at low cost and stored and transported with minimal special handling. Calcium carbonate (commercially available in a number of forms, including limestone) is a material of choice because it meets these criteria and, when properly processed, yields process byproducts that can be easily disposed of as landfill or sold as gypsum.
The design and operation of single-loop, countercurrent spray towers utilizing limestone is discussed by Rader and Bakke, in Incorporating Full-Scale Experience Into Advanced Limestone Wet FGD Designs, a paper presented at the IGCI Forum '91, Sep. 12, 1991, in Washington, D.C. Open spray towers (i.e., those not having packings, trays or other means for facilitating gas-liquid contact) are presented as simple in design and of high reliability for flue gas desulfurization (FGD). The authors do not extensively discuss entrainment separators, but do show a two-stage mist eliminator and describe top and bottom washing.
A thorough discussion of conventional and commercially available mist eliminators is discussed by Jones, McIntush, Lundeen, Rhudy, and Bowen, in Mist Elimination System Design and Specification for FGD Systems, presented Aug. 26, 1993, at the 1993 SO.sub.2 Control Symposium, Boston, Mass. The authors show through extensive testing in a special test rig that high, vertically-upward gas velocities (i.e., those greater than about 4.5 meters per second) in a spray zone are difficult to effectively demist because of a phenomena known as "breakthrough". Breakthrough occurs when the mist eliminator effectively is flooded with liquid due to inadequate drainage. Various mist eliminator designs experience this breakthrough at different gas velocities, depending on the vagaries of the particular design. However, in general, no mist eliminator is satisfactory in vertically upward gas flow above 4.5 meters per second, and all are at risk to experience breakthrough.
It would be desirable to provide mist elimination which was effective at gas velocities even higher than 4.5 meters per second disclosed by Rader and Bakke. One major supplier of vertical flow, limestone-based FGD systems has a mist eliminator design proven suitable for gas velocities in the spray zone greater than 4.5 meters per second. The design is described most typically by the mist eliminator system at N. V. Provinciale Zeeuwse Energie-Maatschappij's Borsselle Power Station, Unit 12, located in the Netherlands, illustrated quite satisfactorily by Rosenberg and Koch in the Jul. 10, 1989 report from Battelle's Stack Gas Emissions Control Coordination Center Group. The design is based on a horizontal flow mist eliminator oriented circumferentially about and above the vertical flow spray zone. Entrained slurry from the spray zone must travel up, then make a radial turn outwardly to pass through the mist eliminator. The mist eliminator operates at a superficial face velocity much lower than that in the spray zone of the tower, indeed less than 20% of the spray zone gas velocity. Further, the upper portions of the mist eliminator are under-utilized while the lower portions, those closest to the spray zone, handle most of the flue gas and entrained slurry. Such a unit as shown at Borsselle is expensive to construct and maintain. Should the Borsselle design be made shorter to lower the cost, then it could be and should be anticipated that severe roof deposits would occur due to impingement of vertically-directed droplets from the spray zone that did not make the turn into the mist eliminator. So while it is readily acknowledged and common practice to operate horizontal flow mist eliminators in limestone-based FGD service at gas velocities of 4.5 to 6.0 meters per second, the Borsselle design cannot be operated this way because of risk of roof deposits.
Impingement of slurry droplets on surfaces upstream or downstream of mist eliminators in limestone-based FGD systems is not desirable. Impinged droplets that do not disengage as droplets, i.e., are not washed off or slough off under their own weight, will form scale deposits as the dissolved calcium ions precipitate with absorbed sulfite oxidized to sulfate. These gypsum scale deposits will tend to build on themselves, growing at a substantial rate until mechanical forces or their sheer weight causes them to break off. This is a very undesirable situation which can, and has, caused serious damage to spray tower internals and other equipment.
The prior art does not directly address the points necessary to achieve improvements in removal of entrained liquid droplets in FGD scrubber gas streams moving vertically at velocities more than 4.5 meters per second without creating problems of the type mentioned above.